The focusing module of a telescope usually comprises a clamp screw. When people focus the telescope by sliding the drawtube to the position that gives the best image, one can screw the clamp screw to hold the drawtube in a fixed position.
For a telescope of the prior art, the drawtube will be displaced slightly when the clamp screw is screwed to clamp the drawtube. For a long focal length telescope, a slight displacement of the drawtube may cause the target out of the field of view of the telescope.